Wireless power supplies can be realized by means of inductive and/or capacitive near-field coupling. This is used in many RFID systems and wireless battery chargers. A source unit, hereinafter referred to as the base station, generates an electromagnetic alternating field whose radiation unit represents a resonant circuit. By means of such a resonant circuit the alternating field is produced efficiently at relatively low cost, because the driver circuits generate virtually no switching losses (ZVS zero voltage or zero current switching ZCS). The filtering effect of the resonant circuit influences the spectral power density and suppresses harmonics. A generator in the base station drives the resonant circuit with a signal whose frequency and amplitude can be varied to control the transmitted power.
The designated energy equipment to be supplied, hereinafter referred to as the load unit is placed at a distance of a fraction of the wavelength of the alternating electromagnetic field so that the coupling condition between the base station and the load unit becomes optimal. This is achieved by the same polarization, minimum distance and greatest coupling surface. Often, additional soft magnetic materials are used in order to further increase the coupling and/or to direct the field profile.
It is known, that the transmitted power at a given coupling factor can be increased by the use of a resonant circuit in the load unit. Optimally, the load resonant circuit shall be tuned exactly to the frequency of the generated alternating magnetic field, where one has to weight achievable coupling gain versus resulting bandwidth.
In “System description Wireless Power Transfer”, a resonant circuit is shown, which is driven by a generator.
Therein, in a resonant circuit a plurality of inductances are added by switches to the resonant circuit to concentrate the radiated field energy to an area where load units are positioned. Additionally, an upstream voltage or current regulator controls the power supply.
The general disadvantage of wireless power transfer principles using resonant circuits in the base station and/or in the load unit is the resonance frequency detuning. This has been due to component tolerances, component aging, variable coupling and load conditions. This load detuning is undesirable because the impedance of the resonant circuit is frequency selective and a predetermined operating frequency no longer matches the resonant frequency. Consequently the overall efficiency of the driver circuit decreases and thus the power transmission from base station to the load unit. In addition the drive waveforms become more distorted and the driver circuits generate more harmonics.
The well-known network measurement method measures while in an interval, the resonant frequency of the network and then it operates at this frequency, but has no ability to control the network frequency actively. This would be very desirable, because regulations such as EN300330, ITU-REC7003 and RSM2123 specify maximal amplitude values for given frequency ranges (for example 119 . . . 135 kHz). Furthermore country-specific narrow band frequencies exist within a given frequency range which define much smaller limiting values.
Therefore, it is important to control not only the power but also the spectral position of the radiated power.
U.S. Pat. No. 6,586,895 shows how to control an inductor or a higher order network using a variable coupling interval during both half waves of a resonant circuit period.
FIG. 1 shows the main circuit wherein the capacitor CS and the inductance LH form a series resonant circuit. The capacitor CM and the inductor LM are both coupled via the controlled transistors Q1a and Q1b and their integrated body diodes to CS in two part-intervals of the resonant circuit period. The coupling control operates in both half cycles of the resonant circuit period, since the current in CM and LM depends in both directions from the control of the transistors Q1a respectively Q1b. The capacitor CR is not relevant, because it remains short-circuited by the transistor QH and diode DH, or the transistors Q1a and Q1b remain fully open or fully closed when QH is open.
In “Stabilisation the Operating Frequency of a Resonant Converter for Wireless Power transfer to Implantable Biomedical Sensors” a controlled resonant network is proposed in a generator, wherein the load resonant circuit can be controlled by a variable coupling interval (see FIG. 2a). In an LC resonant circuit, a second capacitor is coupled for sub-intervals of the resonant circuit period to the LC resonant circuit (see FIG. 2b). The switch control compares the resonant circuit voltage versus a reference voltage which acts as a control variable and the comparator output represents the switching signal for the coupling switches (see FIG. 2c). This control method works very unreliable, since any change in the resonant circuit amplitude immediately interact with the frequency of the resonant circuit.
Coupled load units (respectively changing load conditions) alter the amplitude of the resonant circuit voltage. Also, often used amplitude control methods to control the power transfer, always result in frequency detuning. In fact, the amplitude control and the frequency control interact with each other, they cannot be controlled independently.
Generating control signals, which are dependent on the controlled output signal is very difficult. The main problem is, that an updated output value instantly changes the input value and thus makes the system unstable.
Another important requirement of a base station, whose frequency can be controlled, is the amplitude fidelity. The amplitude shall remain constant, when the frequency changes. This is especially difficult during the sudden step tuning (frequency hopping), because the impedance of the resonant circuit may vary greatly according to the frequency tuning step size.
In a tunable resonant circuit using controlled interval coupling for its component variation, it is very vital that the coupling interval can be controlled independently from the resonant circuit period and amplitudes.
This means the resonant circuit frequency is only a function of a control variable.
This independence condition ensures stable operation regardless of the quality of the response network. Frequency or phase locked loops should preferably have a short lock time. Thereby, frequency or phase changes in the network are immediately corrected. In addition, a fast control loop is important to manage in a sweep operation an operating frequency range with large time resolution (short residence time).
The following invention describes a method and their detailed implementations to control the frequency or phase of a generator via controlled large signal network.
The following invention describes a method that meets all requirements above and is characterized by a short lock time, quality independent stable operation and high efficiency.